Most of the time a diesel engine operates significantly lean of stoichiometry wherein gases expelled from the combustion chambers are characterized by excess oxygen. Richer air/fuel ratios may be controlled during brief periods for the purposes of particulate or oxides of nitrogen (NOx) trap regenerations where such apparatus are utilized as part of the engine emission control system. Diesel engines may also use exhaust gas recirculation (EGR) in the emission controls to reduce the NOx produced in the diesel engine's combustion process by lowering the effective combustion temperature and reducing the oxygen component of the cylinder charge.
Oxygen concentration in the intake manifold is a key parameter in controlling the make up of the exhaust gases expelled from a combustion chamber. Exhaust gases recirculated back into the intake manifold will vary the oxygen concentration in the intake manifold and, in turn, the oxygen concentration in the intake manifold will affect the oxygen concentration in the combustion chambers established during cylinder filling periods. Therefore, the total pre-combustion trapped charge within the combustion chamber may contain different amounts of oxygen depending on the prevailing intake concentration of oxygen during the cylinder filling period. The amount of oxygen affects both the amount of fuel that can be injected before unacceptable levels of particulate emissions (i.e. smoke) are produced and the level of NOx production.
Combustion controls which rely upon post-combustion oxygen sensing are generally satisfactory for managing steady state or slowly varying oxygen levels. EGR dynamics are therefore limited by the effectiveness of such controls in accounting for rapid changes in EGR levels. Additional factors including intake temperature and pressure also affect the oxygen levels. Intake boosting, such as by turbocharging or supercharging, also have limited dynamics in accordance with the effectiveness of such controls in accounting for rapid changes in boost levels.
Ideally, pre-combustion oxygen sensing in the intake manifold would alleviate much of the dynamic limitations mentioned by providing substantially instantaneous intake oxygen concentration measurements thus accounting for rapid changes in EGR concentrations and intake boost pressures. However, known wide range oxygen sensing technologies are effective at substantially elevated temperatures. Whereas they work well in a high temperature exhaust environment, substantial heat would need to be added thereto to achieve light-off in the much cooler intake environment. A supplemental electrical heater would likely result in an unacceptably high power consumption penalty. Also, known wide range oxygen sensing technologies are effective at substantially ambient pressure levels and require proper pressure compensation to produce accurate oxygen concentration information.